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authorJoel Fernandes (Google) <joel@joelfernandes.org>2020-11-03 17:25:57 +0300
committerPaul E. McKenney <paulmck@kernel.org>2021-01-05 00:22:12 +0300
commit6bc335828056f3b301a3deadda782de4e8f0db08 (patch)
treef65cd1ca05fa86eb721ff05ad21c27f3b38369ce /kernel/rcu
parent5c8fe583cce542aa0b84adc939ce85293de36e5e (diff)
downloadlinux-6bc335828056f3b301a3deadda782de4e8f0db08.tar.xz
rcu/tree: Make rcu_do_batch count how many callbacks were executed
The rcu_do_batch() function extracts the ready-to-invoke callbacks from the rcu_segcblist located in the ->cblist field of the current CPU's rcu_data structure. These callbacks are first moved to a local (unsegmented) rcu_cblist. The rcu_do_batch() function then uses this rcu_cblist's ->len field to count how many CBs it has invoked, but it does so by counting that field down from zero. Finally, this function negates the value in this ->len field (resulting in a positive number) and subtracts the result from the ->len field of the current CPU's ->cblist field. Except that it is sometimes necessary for rcu_do_batch() to stop invoking callbacks mid-stream, despite there being more ready to invoke, for example, if a high-priority task wakes up. In this case the remaining not-yet-invoked callbacks are requeued back onto the CPU's ->cblist, but remain in the ready-to-invoke segment of that list. As above, the negative of the local rcu_cblist's ->len field is still subtracted from the ->len field of the current CPU's ->cblist field. The design of counting down from 0 is confusing and error-prone, plus use of a positive count will make it easier to provide a uniform and consistent API to deal with the per-segment counts that are added later in this series. For example, rcu_segcblist_extract_done_cbs() can unconditionally populate the resulting unsegmented list's ->len field during extraction. This commit therefore explicitly counts how many callbacks were executed in rcu_do_batch() itself, counting up from zero, and then uses that to update the per-CPU segcb list's ->len field, without relying on the downcounting of rcl->len from zero. Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'kernel/rcu')
-rw-r--r--kernel/rcu/rcu_segcblist.c2
-rw-r--r--kernel/rcu/rcu_segcblist.h1
-rw-r--r--kernel/rcu/tree.c11
3 files changed, 7 insertions, 7 deletions
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 2d2a6b6b9dfb..bb246d8c6ef1 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -95,7 +95,7 @@ static void rcu_segcblist_set_len(struct rcu_segcblist *rsclp, long v)
* This increase is fully ordered with respect to the callers accesses
* both before and after.
*/
-static void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v)
+void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v)
{
#ifdef CONFIG_RCU_NOCB_CPU
smp_mb__before_atomic(); /* Up to the caller! */
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 492262bcb591..1d2d61406463 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -76,6 +76,7 @@ static inline bool rcu_segcblist_restempty(struct rcu_segcblist *rsclp, int seg)
}
void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp);
+void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v);
void rcu_segcblist_init(struct rcu_segcblist *rsclp);
void rcu_segcblist_disable(struct rcu_segcblist *rsclp);
void rcu_segcblist_offload(struct rcu_segcblist *rsclp);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 40e5e3dd253e..cc6f379c7ddf 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2434,7 +2434,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
struct rcu_head *rhp;
struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
- long bl, count;
+ long bl, count = 0;
long pending, tlimit = 0;
/* If no callbacks are ready, just return. */
@@ -2479,6 +2479,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
rcu_callback_t f;
+ count++;
debug_rcu_head_unqueue(rhp);
rcu_lock_acquire(&rcu_callback_map);
@@ -2492,15 +2493,14 @@ static void rcu_do_batch(struct rcu_data *rdp)
/*
* Stop only if limit reached and CPU has something to do.
- * Note: The rcl structure counts down from zero.
*/
- if (-rcl.len >= bl && !offloaded &&
+ if (count >= bl && !offloaded &&
(need_resched() ||
(!is_idle_task(current) && !rcu_is_callbacks_kthread())))
break;
if (unlikely(tlimit)) {
/* only call local_clock() every 32 callbacks */
- if (likely((-rcl.len & 31) || local_clock() < tlimit))
+ if (likely((count & 31) || local_clock() < tlimit))
continue;
/* Exceeded the time limit, so leave. */
break;
@@ -2517,7 +2517,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
local_irq_save(flags);
rcu_nocb_lock(rdp);
- count = -rcl.len;
rdp->n_cbs_invoked += count;
trace_rcu_batch_end(rcu_state.name, count, !!rcl.head, need_resched(),
is_idle_task(current), rcu_is_callbacks_kthread());
@@ -2525,7 +2524,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
/* Update counts and requeue any remaining callbacks. */
rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);
smp_mb(); /* List handling before counting for rcu_barrier(). */
- rcu_segcblist_insert_count(&rdp->cblist, &rcl);
+ rcu_segcblist_add_len(&rdp->cblist, -count);
/* Reinstate batch limit if we have worked down the excess. */
count = rcu_segcblist_n_cbs(&rdp->cblist);